Well no subwoofer will go from 20 hz up to 600 hz, by definition it would have to be a woofer.
Planning to amplify each driver separately it is agood idea, but the tone, dinamics a low distortion of a compresion driver like a Radian 950 inside a horn is very different than those of a woofer, sub or not.
Why do you want to use a Horn driver?

Well no subwoofer will go from 20 hz up to 600 hz, by definition it would have to be a woofer.
Planning to amplify each driver separately it is agood idea, but the tone, dinamics a low distortion of a compresion driver like a Radian 950 inside a horn is very different than those of a woofer, sub or not.Why do you want to use a Horn driver?

because of high efficiency, bit watt tube amplifiers could be used.
I like the sound of horn speakers

1- A 15" woofer (Altec 416 - 515 and the likes) in a bass reflex cabinet
2- A 12" woofer, like some pro designs out there... sound horrible.
3- A 10 inch woofer, yes they even try this.
4- A 10 inch midrange and a 15" woofer, the "magic" horn.
5- A 15" woofer inside some kind of horn-bass relfex thing.
6- A 15" woofer inside a folded horn, corner or not.
7- A couple of 12" woofers on a open bafle.

I think you are undervaluing what happens below 600Hz. For one thing, it is the bulk of the power spectrum of music, so it's hard to see why high efficiency is important if it's only above that point. For another thing that is sort of the same thing, most of the fundamental notes in music are 40-1000Hz (and 40-600 Hz is about 80% of the notes in that range), so the "sound" of whatever is covering up to 600Hz is going to be a very significant part of how the overall speaker sounds.

As for making it work, there would be a directivity mismatch and you would have some challenges with time alignment, but you could certainly do it if you really wanted.

Can be done if you really want, with a class A flea power amplifier
for the horn, which will have serious dispersion issues at the top end,
but you have ask how does it give the sort of sound you want ?

A 2x12" 0.5 way is probably best for the bottom end, not simple.
Sensitivity, size, bass extension all have to be balanced.